Plasma high density lipoproteins (HDL) are a major protective factor in atherosclerosis and recently treatments have been devised to increase HDL levels and hopefully reduce atherosclerosis. However, the mechanisms of the protective effect of HDL are incompletely understood. Mutations in ABCA1 result in low HDL and impaired efflux of cholesterol from macrophage foam cells to lipid-poor apoA-I. However, while ABCA1 plays a key role in HDL formation, ABCA1 interacts poorly with HDL particles that form the bulk of plasma HDL. This observation led to our recent discovery that two LXR-induced half-transporters, ABCG1 and ABCG4, mediate cholesterol efflux to HDL particles but not to lipid-poor apoA-I. The central hypothesis of this project is that the activities of ABCG1 and possibly ABCG4 mediate the protective effect of HDL by promoting the efflux of cholesterol from macrophages and other vascular cells to HDL. In Aim 1, we will evaluate the cellular mechanisms of cholesterol efflux mediated by ABCG1/4 and a potential role of ABC transporters in preventing cholesterol-induced macrophage apoptosis. In Aim 2 the role of ABCG1 and ABCG4 in vivo will be evaluated in gene knock-out mice and effects of transporters on atherogenesis will be evaluated in apoE and LDL R KO backgrounds. In Aim 3, the function of ABCG1 in specific cell types in atherosclerotic lesions will be evaluated by development of conditional knock-outs. The specific roles of ABCG1 and ABCA1 in HDL protection will be evaluated by seeing if the protective effect of increased HDL levels resulting from apoA-I overexpression is reduced in a setting of macrophage ABCG1 and/or ABCA1 deficiency. This project will evaluate the mechanisms and in vivo significance of new cellular cholesterol efflux pathways involving ABCG1 and ABCG4, and may lead to new therapeutic concepts, such as combined treatments to both increase HDL levels and to up-regulate ABC transporters in arterial wall cells. [unreadable] [unreadable]